This invention is related to xerography and more particularly to a developing device for rendering latent electrostatic images visible.
Conventional xerographic developing techniques such as cascade development, magnetic brush development, etc. require the use of a composite developing material which is a mixture of so-called carrier material and toner particles. By rubbing in contact with the carrier, the toner particles acquire an electrostatic charge of a given polarity which is opposite to that of the electrostatic latent image so that it becomes attracted by the latter.
The fact that a toner/carrier mixture is used in these conventional techniques and that only one component of this mixture namely the toner, is gradually consumed during development, necessitates that replenishment must be carried out with fresh toner only.
In order to precisely dose the exact amount of toner powder, relatively complicated systems have been devised, which however fail to provide the necessary guarantee that replenishing is carried out optimally. As a consequence, the replenished toner/carrier mixture may be over or underregenerated, so that optimum conditions for reproducible copying runs are very difficult to create.
It has therefore been proposed e.g. in Belgian Pat. No. 848,236 filed Nov. 10, 1976 by Escophot, to use the toner developing powder alone so that a carrier material is no longer required. In this way replenishment is very easy to realize as the replacement of used toner by fresh one independent of the mass of carrier, may be carried out without complications.
Development occurs by bringing the toner powder into contact with the electrostatic latent imge. Prior to developing, a charge of given polarity is conferred to the toner particles by rubbing them against rollers, scrapers or other expedients having to fulfil certain requirements--especially in tribo-electrical characteristics--so that the toner particles can be deposited on oppositely charged surfaces. This manner of toner deposition is in the art known as "touch-down" or "transfer" development.
The nature of the preliminary charging process is such that in order to get a defined tribo-electric relationship between the toner particles, the applicator body and the walls of the tray in which toner powder is contained, the materials of construction for the aforementioned mechanical parts have to possess adequate tribo-electric characteristics versus the toner powder. So, an applicator roller will have an electric resistivity of about 10.sup.4 Ohms/square, whereas a scraper mounted as a toner flow regulating doctor blade will be made of a material which is a dielectric in nature. Under such conditions, it becomes possible to confer a charge of a given polarity to the toner particles.
Unfortunately, this charge is of rather moderate magnitude. Practical values of it fall between 0.2 to 0.5 micro Coulombs per gram (details about the measurement of this parameter will be given further).
As a consequence of the rather low charge acquired by the toner prior to its deposition onto the latent electrostatic image, this manner of development shows the following inconveniences: high contrast images, high densities and a very steep gradation. This is primarily due to the fact that a large quantity of toner is required in order to neutralize the charge of the latent electrostatic image on the image forming medium.